I. Field of the Invention
This invention relates generally to tungsten carbide insert rock bits and tools of the rolling cutter type and more particularly to the specially shaped and designed inserts utilized thereon.
II. Description of the Prior Art
Rock bits using sintered tungsten carbide inserts generally have a wedge or chisel-shaped configuration for soft to medium hard formations. Various embodiments of such configurations are shown in U.S. Pat. Nos. 3,442,342 and 4,108,260. Such chisel-shaped inserts conventionally have a cylindrical base for retention and a wedge-like projection cutting tip. In all forms of chisel type inserts, flanks are made into the insert by removing material from two opposing sides of a truncated cone. A curvilinear crest formed from a tip radius connects to the top of the two flanks. The remaining truncated cone, a conical surface symmetric with the insert's axis, is joined to the crest with two opposing corner radii that are revolved about the insert's axis. The intersection between the corner radii and the tip radius as well as the intersection between the conical surface and the flanks usually have a blending radius, also known as a round, to eliminate the sharp edge that would otherwise exist.
Blend radii are areas of high stress concentration due to that they are small, typically 0.005 to 0.090 of an inch. These small blend radii typically contribute to or cause the premature breakage of an insert, of which in many cases a less optimum material composition is utilized to overcome this failure mode.
The performance of an insert is primarily dictated by its cutting tip's ability to penetrate into the required formation without having a failure due to breakage or in some cases excessive wear. The tip radius size is predominantly constrained by the desired sharpness curve of an insert cutting tip. The sharpness curve is a plot of the projected cutting tip cross sectional area versus the distance from the top of the cutting tip. To enable sufficient penetration of an insert into a formation the desired sharpness curve is generally obtained by utilizing a small tip radius on the order of 0.075 to 0.125 of an inch.
The conical surface, regardless of whether the flanks are planar, convex, concave, or spherical, or whether the crest intersects the insert axis or is offset from the axis, as shown in the above mentioned patents, is a surface of revolution about the insert axis that adds non-functional material to the insert cutting tip. This additional material adds to the constraint of reducing the tip radius to obtain the desired sharpness curve, which in turn increases the stress concentration on the insert crest and reduces the durability of the insert.
The blend radius on an insert is limited in size by the tip radius, which requires it to be the same size or smaller. In many cases small tip radii will wear away quickly until the initial crest is completely gone. With the crest gone, the blend radii between the conical surface and the flanks is exposed on the cutting surface providing areas of high stress concentration. As the insert tip wears, the surface area of the cutting surface increases, thus the rate of wear decreases which exposes areas of blend radii to more cutting cycles and fatigue leading to chipping and breakage resulting in insert failure.
The sharp edge intersections requiring blending are extremely complex three-dimensional paths. Due to this complex path, the blend radius used on an insert is put into the tooling mold by hand and is typically not controlled from one set of tooling to another, thus a wide variance of blend radii can occur from one production to another of the same insert model which in turn can produce different performance results of one rock bit to another of the same rock bit model.